1. Field of the Invention
The present invention relates, in general, to a catalyst for denitrification and, more particularly, to a catalyst for removing nitrogen oxides (hereinafter referred to as NOx) from flue gas, which is of excellent catalytic activity and of high resistance to SO2 poisoning as well as is economically favorable in its production. Also, the present invention is concerned with a method for preparing such a catalyst.
2. Description of Prior Arts
NOx are generally produced from high-temperature combustion process as a result of the oxidation of nitrogen compounds contained in fuels and/or the reaction of the nitrogen and oxygen excessively fed from the air to the facility. In addition to being a source of photochemical smog, NOx, when exhausted into the air, is known as a main cause of acid rain, along with SO2. Thus, extensive effort has been made to develop technologies for the removal of the pollutant from flue gas.
In order to reduce NOx emitted from the stationary sources, combustion modification methods, such as low NOx burners, flue gas-circulating techniques, etc. have been commonly employed. However, combustion modification methods, although NOx removal efficiency varies with the technologies applied, can not achieve more than 50% of NOx removal efficiency, in general. As an effective method to reduce NOx emission, flue gas denitrification has recently attracted the industrial attention.
Flue gas denitrification processes are largely classified into wet methods using absorption solutions and dry methods using adsorption, catalytic decomposition and/or catalytic reduction. Compared with the dry methods, the wet methods are economically and technically unfavorable, since they require large installation and operating costs and can produce secondary waste water which should be treated for discharge. Among the dry methods, the most widely employed technology is the selective catalytic reduction (hereinafter referred to as SCR ), which is the catalytic reduction of NOx into harmless N2 and H2O by reductant, NH3. The backbone of SCR technology is a highly active and durable catalyst. Various catalysts, including precious metals, metal oxides, zeolites, etc. have been suggested as being useful for SCR. Among them, vanadium pentoxide-based and zeolite-based catalysts are known to be excellent in its performance.
Vanadium pentoxide-based catalysts show the variety of catalytic activity, depending on the supports which include typically titanium dioxide (TiO2), aluminum oxide (Al2O3) and silicon dioxide (SiO2). However, titanium dioxide is commonly employed as the support of SCR catalyst due to the catalytic activity for NOx removal and durability under actual flue gas conditions (T. Shikada, K. Fujimoto, T. Kunugi and H. Tominaga, J. Chem. Tech. Biotech., 33A, 446(1983); H. Yoshida, K. Takahashi, Y. Sekiya, S. Morokawa and S. Kurita, Proc. 8th Conf. On Catal., Berlin, III-649(1984)).
These vanadium pentoxide-based catalysts can be prepared by well-known precipitation methods (H. Miyata, K. Fujji, T. Ono and Y. Kubokawa, J. Chem. Soc. Fara. Trans., 83, 675(1987); M. Sanati, L. R. Wallenbweg, A. Andersson, A. Jansen and Y. Tu, J. Catal. 133, 128(1991)) or by mixing the solution containing vanadium precursor with support materials at desired weight ratios and followed by drying and calcining the mixtures as disclosed in Japanese Pat. Nos. 59-35027, 58-210849 and 58-183946.
In addition to the high performance of NOx removal activity, the vanadium pentoxide-based catalysts should be also highly resistant to the poisoning of the catalyst by SO2. In general, SCR catalyst can be easily poisoned by SO2 which is contained at a level of several hundreds of ppm in flue gas from fossil fuel-burning boilers. Therefore, to remove NOx from flue gas containing SO2, the catalysts are required to be highly resistant to SO2 poisoning as well as superior NOx removal activity.
Conventionally, prior to SCP process, a wet flue gas desulfurization(FGD) has been installed to eliminate sulfur oxides and thereby to prevent NOx removal catalysts from being deactivated in catalytic activity due to the existence of SO2 contained in the flue gas as well as NOx. Although it can protect SCR catalyst from the poisoning by SO2, it can not be a fundamental measure. The operating cost of SCR process can not be ignored due to the temperature difference between FGD and SCR processes, mainly heating cost of flue gas from the temperature of exhaust stream of FGD process to that of the operating temperature of SCR process. Moreover, large investments are also needed to install and operate FGD process.
The catalyst poisoning by SO2 is mainly attributed to the following two reasons. The one is the plugging of catalyst pores by the ammonium salts, which are formed through the reaction of SO3 with the reductant, NH3. The other is the pore blocking and/or the decrease of active sites of the catalyst by the reaction of SO3 with the support and/or the active components of the catalyst. Accordingly, the low conversion of SO2 oxidation to SO3 is required for the durability of a catalyst over SO2. In addition to the catalyst poisoning, SO3 produced by SO2 oxidation, can also react with unreacted ammonia forming ammonium salts which cause the operational problems of the SCR processes such as corrosion and plugging of the downstream of the SCR reactor. Therefore, an useful SCR catalyst should have low catalytic activity for SO2 oxidation to SO3, which largely depends on the chemical composition of the catalyst.
The durability of a catalyst against SO2 also affects the arrangement of the processes for treating flue gas. For the SCR process where the catalyst is readily poisoned by SO2, FGD process should be provided before the SCR process to eliminate SO2 in advance. In this case, the flue gas at high temperature should be cooled to lower temperature (e.g., 50xc2x0 C. or lower) adequate for effective SO2 removal, and then the gas exhausted from a desulfurizing system should be reheated to a temperature (e.g., 300xc2x0 C. or higher) at which the reaction for NOx removal can effectively take place. This is a main disadvantage of the SCR process in terms of energy efficiency. In contrast, SCR catalyst with the high durability against SO2 allows the direct treatment of the flue gas in the process without the previous removal of SO2 by FGD. Therefore, additional energy cannot be consumed to reheat the flue gas. Since the sulfur tolerance of the catalyst has a great influence on not only the life span of the catalyst itself, but also the energy efficiency of the SCR process, it is one of the most important catalytic properties determining the economics of the process.
According to the present invention, the highly active and durable catalyst for NOx removal can be prepared by the catalyst preparation methods such as the impregnation and adsorption of vanadium pentoxide on titanium dioxide containing the pore size ranging from 500 to 70,000 xc3x85. The titanium dioxide support could be obtained from a metatitanate (TiO(OH)2)-predominating intermediate in a form of slurry which is produced during the course of the production of titanium dioxide for pigment from ilmenite.
Therefore, the objective of the present invention is to overcome the problems mentioned earlier and to provide a highly active catalyst for removing NOx from flue gas.
It is another objective of the present invention to provide a catalyst for removing NOx from flue gas, which maintains an extended life span without losing NOx removal activity.
It is a further objective of the present invention to provide a catalyst for removing NOx from flue gas, which can be produced at low cost of the preparation.
It is still a further objective of the present invention to provide a method for preparing such a catalyst.
In accordance with an aspect of the present invention, a V2O5-based catalyst is provided for removing nitrogen oxides, comprising vanadium pentoxide, and barium oxide or calcium oxide as active components on a titanium dioxide support. The titanium dioxide support can be prepared by sufficiently drying and calcining a metatitanate (TiO(OH)2)-predominating slurry which is obtained from the production process of titanium dioxide for pigment from ilmenite.
In accordance with another aspect of the present invention, a preparation method of a V2O5-based catalyst is provided for removing nitrogen oxides, which comprises the following steps of: sufficiently drying a metatitanate (TiO(OH)2)-predominating slurry to dehydrate therefrom. The slurry is an intermediate product obtained during the course of the production of titanium dioxide for pigment; calcining the dehydrated slurry at 400xcx9c700xc2x0C. for at least 5 hours.; and loading vanadium pentoxide, along with barium oxide or calcium oxide, on the titanium dioxide, serving as a catalyst support.
A highly active and durable catalyst is obtained by the synergistic action of vanadium pentoxide with barium oxide or calcium oxide on a titanium dioxide support. For the catalyst developed in the present invention, vanadium pentoxide plays a role as a catalytically active ingredient for NOx removal from flue gas, while barium oxide or calcium oxide serves as an inhibitory component against the catalyst poisoning by SO2. The titanium dioxide support is prepared from a metatitanate (TiO(OH)2)-predominating slurry obtained from the production process of titanium dioxide for pigment. This slurry is dehydrated and calcined to prepare a titanium dioxide support with the pore size, ranged from 500 to 70,000 xc3x85. On the surface of this support is uniformly dispersed vanadium pentoxide, along with barium oxide or calcium oxide, through impregnation or dipping methods. The dipping is better than the impregnation method in terms of the dispersion of active components on the catalyst surface.
To prepare highly active vanadium pentoxide-based catalyst supported on a titanium dioxide for NOx removal, the hydrogen ion concentration (pH) of the precursor solution for vanadium pentoxide must be carefully controlled during the preparation of the catalyst. An available pH range for the precursor solution is 2 to 6 and preferably 2 to 3.
As the catalyst for NOx removal using ammonia as a reductant, vanadium pentoxide is preferably loaded on the catalyst surface of TiO2 at the amount of 1xcx9c10 weight %, based on the total weight of the catalyst, that is, titanium dioxide, vanadium pentoxide, and barium oxide (BaO) or calcium dioxide (CaO). For instance, when vanadium pentoxide is loaded less than 1.0 weight % on the support, very low NOx removal efficiency can be obtained. On the other hand, when the loaded amount of vanadium pentoxide exceeds 10 weight %, ammonia, serving as a reductant in this catalytic system, is so actively oxidized that NOx removal activity significantly decreases at the reaction temperature above 400xc2x0 C. In addition, the catalyst containing vanadium pentoxide more than 10 weight % promotes SO2 oxidation reaction by which SO3 is produced and causes significant reduction of the catalytic activity. Moreover, the oxidation of SO2 results in producing SO3 which can further react with unreacted ammonia at the downstream of a reactor in combustion equipment. As a result of this reaction, ammonium salts are produced and deposited at the downstream of a reactor, causing the plugging and corrosion of the facility. Accordingly, when flue gas contains SO2, it is not preferable to increase the content of vanadium pentoxide higher than 10 weight % on the catalyst surface.
In order to reduce the catalytic activity for the oxidation of sulfur dioxide to sulfur trioxide, which are exhausted along with NOx, barium oxide or calcium oxide is added to the catalyst containing vanadium pentoxide. This active component is particularly useful to suppress the formation of SO3 from SO2. Its loading amount on the titanium dioxide support is in the order of 1 to 8 weight %, based on the total weight of the catalyst and preferably 3 to 5 weight %. For instance, if less than 1 weight % of barium oxide or calcium oxide is added to the catalyst, the catalyst poisoning by SO2 is not altered. On the other hand, if too much barium oxide or calcium oxide higher than 8 weight % is added, NOx removal activity significantly decreases due to the low dispersion of vanadium pentoxide on the catalyst surface.